Caledonian nappe geometry in north-central Sweden and basin evolution on the Baltoscandian margin

Abstract

AbstractStructural analysis of the Lower Allochthon in the north-central Scandinavian Caledonides has allowed the construction of restorable cross-sections consistent with the development of a foreland-propagating linked thrust system. The internal geometry of an antiformal stack, the Njakafjäll duplex, within the Lower Allochthon demonstrates tectonic shortening of c. 50% and suggests an overall predeformational width for the Lower Allochthon in this area of at least c. 130 km, and possibly considerably greater if the buried trailing edge of the Lower Allochthon lies in a comparable position to that farther south in Tröndelag. These results, combined with a stratigraphic analysis of the imbricates within the Lower Allochthon and of the adjoining Autochthon and Middle Allochthon, indicate the development, from Proterozoic through Cambrian times, of two sedimentary basins on the c. 200 km wide continental margin of Baltica bordering the Iapetus Ocean. The basins were separated by a region of basement relief, the Børgefjell domain, above which a reduced sequence of Vendian to Cambrian rocks accumulated. This Børgefjell basement high, and the similar Njakafjäll basement high to the east, subsequently became the sites of antiformal stack development. It is argued that the frequent incorporation of basement into the thrust sheets, together with the thin sedimentary fill of these basins, compared with the much greater fill in basins to the south in Jämtland and to the north of Finnmark, implies major palaeogeographic changes along the Baltoscandian margin, possibly related to early rift geometries. The apparent lack of subsequent foreland basin development in north-central Scandinavia compared with areas to the south may indicate a deeper level of thrust detachment beneath the Middle Allochthon to the north, such that any foreland basin sediments have been removed in the hangingwall and subsequently eroded. An alternative possibility is a primary absence of foreland basin development that may relate to a differing response to thrust loading by continental lithosphere which had been variably thinned during the earlier rift regime.